The invention relates to an apparatus and method for the digestion of cellulosic fiber material (including sawdust, fines, pinchips, and other small material particles) providing the most intimate and uniform heating and mixing possible.
Two-vessel digestion of cellulosic fiber material is known, in order to facilitate uniform treatment of the pulp and for shortening digestion time after impregnation, as shown in U.S. Pat. No. 3,802,956. However, such prior two-vessel digestion has several areas thereof which may be improved upon. For instance, such prior arrangements have a mechanical separator disposed at the top of the second vessel, steam lines leading into the second vessel, and separate heating circuits for both the impregnation vessel and the digesting area of the digestion-washing vessel, and a fair amount of other equipment which can raise the costs of installation and maintenance for such a facility significantly. Intimate heating and mixing of the liquid and fiber material -- while facilitated -- is not ideally provided, and there are portions of the fiber material in the digestion-washing vessel which become non-uniformly treated since (depending upon the pile configuration in the vessel) they are out of the liquid, and exposed to steam at the top of the digestion-washing vessel, for a longer period of time than other adjacent portions of the fiber material column. Also, it is difficult to provide uniform heating and liquor flows in the digesting vessel when large quantities of sawdust and other fine material make up the cellulosic material being treated.
According to the apparatus and method of the present invention, the above-mentioned problems are avoided, and in general complete intimate heating and mixing are provided, resulting in pulp of very uniform quality. According to the present invention, the mechanical separator and accessory steam lines, heating circulatory loops, and other structures of the prior arrangement of U.S. Pat. No. 3,802,956 are eliminated while as good or better ultimate treatment is provided. Since the between-vessel transfer and heating lines are the same, much of such equipment in the prior arrangements may be eliminated, while providing a longer period of time in which the heated liquid and fiber material are in intimate contact with each other. Since the digestion-washing vessel is liquid-filled, no non-uniformities are introduced no matter what the pile configuration at the top of the digestion-washing vessel. Also, uniform treatment is provided despite the quantity of sawdust and other fine materials which is provided in the make-up of the fiber material being treated.
According to the present invention, a method of digesting cellulosic fiber material utilizing first and second separate vertical treatment vessels is provided, the method comprising the steps of continuously feeding cellulosic fiber material entrained in treatment liquid into a top portion of the first vessel, establishing a first flow path of cellulosic fiber material entrained in and impregnated with treatment liquid from the bottom portion of the first vessel to a top portion of the second vessel, liquid substantially filling the second vessel, establishing a column of fiber material in the second vessel below the level of liquid in the second vessel, withdrawing liquid from the top portion of the second vessel, establishing a second flow path of the liquid withdrawn from the second vessel back toward the bottom portion of the first vessel, heating the liquid during transport in the second flow path, feeding a portion of the heated liquid flowing in the second flow path into the first path while feeding the rest of the heated liquid to a bottom portion of the first vessel, and withdrawing digested fiber material from the bottom of the second vessel. The withdrawal of liquid from the top portion of the second vessel is preferably accomplished without screening, a "stilling well" being provided at the top of the second vessel above the level of fiber material in the second vessel. Washing liquid may be introduced into the second vessel to effect washing of the digested pulp therein, before withdrawal of the digested pulp from the second vessel. A third flow path may be established of treatment liquid from a lower portion of the first vessel back into the first vessel to establish countercurrent flow of liquid upwardly in the first vessel, the withdrawn liquid being heated in the third flow path. The flow of liquid in either the second or third flow path may be supplemented by fresh digesting liquid. Withdrawal of liquid from the first vessel in the third flow path also may be accomplished without screening, a "stilling well" being provided in an enlarged bottom portion of the first vessel.
According to the apparatus of the present invention, apparatus for the digestion of cellulosic fiber material is provided comprising a first vertical treatment vessel, a second vertical treatment vessel separate from the first vessel and being substantially liquid-filled, means for feeding cellulosic fiber material entrained in treatment liquid to the top portion of the first vessel, means for establishing a first flow path of cellulosic fiber material entrained in and impregnated with treatment liquid from the bottom portion of the first vessel to the top portion of the second vessel, a column of fiber material being established in the second vessel below the level of liquid in the second vessel, means for establishing the second flow path of liquid withdrawn from the top portion of the second vessel back toward the bottom of the first vessel, means for heating the liquid in the second flow path, means for feeding a portion of the heated liquid in the second flow path into the first flow path while feeding the rest of the heated liquid to the bottom portion of the first vessel (the bottom of the first vessel providing the heating chamber), and means for withdrawing digested fiber material from the bottom of the second vessel. The heating chamber provided in the bottom of the first vessel along with the relatively long first and second flow paths provide for intimate mixing and heating of the liquid and entrained fiber material before the fiber material is fed into the top of the digester-washing vessel. Since the between vessel transfer line and the heating line are one in the same, no accessory circulatory and heating loops or the like are necessary. The apparatus may also comprise means for establishing a third flow path of treatment liquid from a lower portion of the first vessel back into the first vessel to establish a countercurrent flow liquid upwardly in the first vessel, and means for heating the liquid in the third flow path. Preferably, the liquid withdrawn to the second and third flow paths is withdrawn without screening, "stilling wells" being provided at the top of the second vessel and at the top of an enlarged bottom portion of the first treatment vessel.
The primary object of the present invention is to provide a method and apparatus for effecting completely intimate and uniform heating and mixing of fiber material and digesting liquid to provide digested pulp of uniform quality. This and other objects of the invention will become apparent from an inspection of the detailed description of the invention and from an inspection of the appended claims.